The present invention is directed to the fields of energy storage device (ESD) state of charge management, energy storage temperature control, electrical distribution grid demand charge management, and related fields.
With the electrical demand of a customer in today's electrical distribution grid rising at a restless pace, electricity providers have had to find ways to prevent or discourage overloading of the transformers, feeders, and mains across their distribution networks. In some areas, utilities have adopted the practice of having two separate components in their charges, including a component for overall electrical usage in kilowatt-hours and a component for peak demand. Peak demand billings, called “demand charges,” are electricity bills assessed due to the highest consumption drawn by a customer during a billing cycle. When calculating demand charges, utility providers typically measure facility energy usage over short predetermined time periods (e.g., every ten or fifteen minutes), calculate and store the average level of demand for each of these periods, and then, at the end of a billing cycle (e.g., at the end of each month), generate the demand charge billed to the customer based on the highest consumption average experienced in that cycle. Thus, even brief spikes in demand may result in high demand charges.
Demand charge management is the practice of reducing demand charges. It usually involves reducing the peak electrical load drawn at a site at times when utility costs are high or when consumption at the site is high, thereby reducing the averaged demand across the entire billing cycle or subdivisions thereof. Consumers have used peak mitigation and peak shaving techniques to manage demand charges, wherein an ESD is discharged to the grid or loads are shed from the grid (e.g., turned off, throttled, or diverted to other energy sources) when a spike or peak in the electrical demand occurs in order to offset or nullify the contribution of the peak to the demand averages calculated by the utility provider.
In recent years, electric vehicle charging equipment has become a common source of these peaks, since they charge EVs for relatively brief periods of time at relatively high power levels when compared to the consumption “noise” of lesser-powered devices turning on and off at a site. Therefore, ESDs are implemented to provide charging power to the EVs for short periods of time and then the ESDs are recharged from the electrical grid at a slower rate, thereby keeping short-term demand-averaged consumption lower than it would be if the EVs were charged directly from the grid. In these and other demand charge management settings, energy storage devices are exposed to low temperatures in large numbers. For example, when batteries are used as the energy storage devices for a demand charge management device, the large number of cells, large enclosures, and other equipment needed can force the user to store the batteries outside. Unfortunately, battery chemistries in today's electric vehicles (EVs) and commercial energy storage systems are often sensitive to the cold and may even become fire hazards if they are quickly charged or discharged at low temperatures.
Batteries and electronic hardware generate heat when they are being used due to the inherent exothermic properties of the systems. For example, typical energy efficiency for a round trip discharge-recharge cycle of a lithium-ion battery is about 97%, so approximately 3% of the energy used is turned into heat. However, energy storage systems such as batteries that are used for demand charge management may undergo extended periods of inactivity, during which time they are not generating heat. Existing technology uses heaters such as space heaters or a heat transfer medium to keep the energy storage devices' temperatures from dropping to dangerous levels, but they are an inefficient use of space in the tight enclosures, generate too much waste heat, and are counterproductive in demand charge management installations where electrical demand on the grid must be minimized. Additionally, use of space heaters drives up daily electricity usage and peak electricity usage, which can be counterproductive to the goals for which peak mitigation energy storage devices are installed in the first place.